Kinesins are eukaryotic cytoskeletal proteins best known for their motile activity, but they are also important regulators of microtubule (MT) dynamics, the ability of the MT polymer to grow or shrink. This project seeks to establish the molecular mechanism of microtubule (MT) dynamics regulation by members of the kinesin superfamily. Regulation of MT dynamics by these kinesins play important roles in a variety of cell processes such as mitosis, cytokinesis, neural development and control of cilia and centriole length, but the mechanisms by which these kinesins stabilize or destabilize MTs are not fully clear. We will test the general hypothesis that all these kinesin affect MT-dynamics by modifying the structure of tubulin to favor or disfavor the formation of the inter-tubulin contacts that hold the MT polymer together. To address this hypothesis we will use electron microscopy and other approaches to determine the structure of tubulin complexes of three model kinesins. The proposal has two general aims. The fist aim is to establish the mechanism of MT depolymerization by the kinesins KLP10A and Kar3. The second aim addresses the mechanism of MT stabilization by the kinesin KIF14.